While most 6-a-side football games are generally won by the team with the highest fitness levels (because they can chase the ball more and track back into defense a lot quicker) and the team with the best football skills (since they can dribble round opponents better and pass the ball more sharply), tactics can play a huge part in determining the output. Improving your tactics is one of the best ways to upset the odds, and is the best way to improve your chances of winning if you're the clear underdogs.
In a game where both teams' ability is identical, the more tactically astute team will always win. That being said, it is no good simply knowing about tactics if you can not (and do not know how to) implement them.
Since you are only playing with 5 men (plus one goalie), the best tactics that I have found (through several years of playing 6-a-side football) are to play a 2-1-2 formation. That means that you have 2 men in defense (that rarely, if ever stray from their positions), 1 man in midfield (who links the play while providing support for the attack and cover for the defense) and 2 strikers up front.
The strikers also play a very important part in defending, since they must 'defend from the front'. This means that whenever the opposing team's backline has the ball they need to put them under pressure so that they have no time or space to pick a pass to one of their attackers.